calculate index in the FsExport constructor

While debugging slow NFS performance accompanied by high CPU usage noticed that FsExport.getIndex() determines some index on every call from what seems like immutable data. Moved it to constructor.

Target: master
Request: 0.7
Acked-by: Paul Millar <paul.millar@desy.de> Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7372/